It is known in the art that the sintering of glass-ceramic cordierite articles formed from glass powders of stoichiometric composition 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 is very poor in that porous, granular, mechanically weak products are produced. Various methods for improving the sinterability of cordierite glass-ceramics, while maintaining its favorable qualities of high refractoriness, low coefficients of thermal expansion and excellent thermal insulating properties, have been disclosed in the prior art.
U.S. Pat. No. 3,450,546 describes the manufacturing of transparent cordierite glass-ceramics from glass powders by utilizing brief sintering treatments followed by consolidation-crystallization treatments at temperatures in the 1200.degree.-1425.degree. C. range. This method, however, was found to severely restrict the utility of cordierite compositions for use in preparing complex articles, coatings or seals. U.S. Pat. No. 3,926,648 discloses a method of improving the sinterability of cordierite glass-ceramic compositions by additions of minor amounts of K.sub.2 O and/or Cs.sub.2 O (0.3-1.5 mole percent) to glasses near the cordierite stoichiometry. In addition, U.S. Pat. No. 4,015,048 discloses additions of controlled amounts (0.7-5.6 mole percent) of one or more modifying oxides selected from the group consisting of BaO, PbO, SrO and CaO, to improve the sinterability of glasses having a cordierite composition.
The addition of minor amounts of manganese oxides (3-4 wt.%), to mineral raw batch materials having a cordierite composition in order to lower the firing temperature and increase the sinterability of such mineral batches is described in British Pat. No. 888,227. This process, however, required the addition of a fluxing agent which had the negative effect of increasing the coefficients of thermal expansion of the fired body. U.S. Pat. No. 3,885,977 discloses and claims an anisotropic polycrystalline sintered ceramic product, having cordierite as its primary phase in which as much as 98% of the MgO can be replaced by MnO in the raw batch. This patent, however, fails to disclose or suggest the forming of an impervious void-free product as a result of such replacements.